The present invention relates to a process for the very careful checking of the print quality on deformable materials, such as sheets of paper, plastic or rubber. More precisely, the invention constitutes a process for modifying, in real time during inspection, the reference models customarily used in automatic checking of print quality so as to correctly inspect, even in cases where the deformation of the sheets produces distortions such that the printed shapes turn out very differently from the model, but nevertheless are still acceptable to the human eye. In fact, all the processes known hitherto use rigid models (undeformable), and hence, in order to compensate for the deformations of sheets (paper, plastic, rubber), are compelled to considerably relax the tolerances so as to reduce the risk of false detections. In particular, this produces a very often unacceptable reduction in the carefulness of inspection.
Several processes for judging the quality of print are generally known: a few examples are given in the list of references. Although several authors have proposed a great variety of setups, almost all the solutions are based on the same fundamental approach, which can be aptly summarized as follows.
A set (Training Set, TS) of one or more samples of objects (sheets, etc.) with a xe2x80x9cgoodxe2x80x9d quality of print is used to xe2x80x9ctrainxe2x80x9d the inspection system using the reference model, and (or) the relevant printing tolerances in terms of densitometric measurements. In general the process consists in capturing, by means of an electronic scanning system, image(s) of sample(s) of the TS and of the construction of a xe2x80x9creference modelxe2x80x9d (sometimes referred to as a xe2x80x9cgolden templatexe2x80x9d) which may be the average of the TS images (or some desirable transformation of them). In addition to the densitometric reference value (that is to say the value in the reference image), for each pixel of the analyzed image, a pair of limit values is calculated (for example, too dark TD and too light TL). Several techniques have been proposed to extract these limits: for example some authors use the minimum (over TS) of the densities of the pixel as TD, and its maximum as TL; others use the gradient of the image; others the standard deviation; etc. In any event, the reference model is a description of the printing tolerances which associates the densitometric limits TD and TL with each pixel of the image. These descriptions are xe2x80x9crigidxe2x80x9d, that is to say there is no possibility of compensating for deformations, which produce a relative displacement of the printed structure. Therefore, in all the previous solutions, despite the introduction of a few clever processes for relaxing the tolerances of the thresholds (TD and TL), the deformations of the carrier (paper, plastic, etc.) are the main source of the detection of xe2x80x9cfalse defectsxe2x80x9d, that is to say, of prints which do not have defects as far as a human inspector is concerned, and which, notwithstanding this, are rejected by the system. In addition, an increase of this kind in the tolerances causes the inspection to become rather rough and inaccurate, consequently reducing the standard of quality. Therefore, what is needed is a method which enables the automatic and accurate inspection of print quality of deformable objects, without sacrificing the standard of quality.
The invention is a process which solves the problem identified above by producing, through electronic means, a model for automatically inspecting the print quality on deformable objects. The model is firstly produced by capturing with an electronic camera (CCD for example) the images of a set of sheets whose print quality is regarded as acceptable; the images are stored so as to produce a first reference image, together with the relevant densitometric tolerance limits. This reference image is thereafter divided into a multitude of sub-images by superimposing a grid with very small mesh cells. During inspection, the distances between the nodes of the grid are measured on the image to be inspected: this therefore produces an elastic modification of the model, which is such as to make the distances between the nodes the same as in the image to be inspected. The image to be inspected is thus verified with respect to the modified reference (model) by using any of the standard inspection techniques.